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INTRODUCTION:
A ferroelectric memory cell consists of a ferroelectric capacitor and a MOS transistor. Its construction is similar to the storage cell of a DRAM. The difference is in the dielectric properties of the material between the capacitor's electrodes. This material has a high dielectric constant and can be polarized by an electric field. The polarisation remains until it gets reversed by an opposite electrical field. This makes the memory non-volatile. Note that ferroelectric material, despite its name, does not necessarily contain iron. The most well-known ferroelectric substance is BaTiO3.
A Ferroelectric memory cell consists of a ferroelectric capacitor and a MOS transistor. Its construction is similar to the storage cell of a DRAM. The difference is in the dielectric properties of the material between the capacitor's electrodes. This material has a high dielectric constant and can be polarized by an electric field. The polarisation remains until it gets reversed by an opposite electrical field. This makes the memory non-volatile.Data is read by applying an electric field to the capacitor. If this switches the cell into the opposite state (flipping over the electrical dipoles in the ferroelectric material) then more charge is moved than if the cell was not flipped. This can be detected and amplified by sense amplifiers. Reading destroys the contents of a cell which must therefore be written back after a read. This is similar to the precharge operation in DRAM, though it only needs to be done after a read rather than periodically as with DRAM refresh.
FRAM is found mainly in consumer devices and because of its low power requirements, could also be used in devices that only need to activate for brief periods. FRAM allows systems to retain information even when power is lost, without resorting to batteries, EEPROM, or flash. Access times are the same as for standard SRAM, so there's no delay-at-write access as there is for EEPROM or flash. In addition, the number of write cycles supported by the FRAM components is nearly unlimited—up to 10 billion read/writes. FRAM combines the advantages of SRAM - writing is roughly as fast as reading, and EPROM - non-volatility and in-circuit programmability
FRAM Technology
When an electric field is applied to a ferroelectric crystal, the central atom moves in the direction of the field. As the atom moves within the crystal, it passes through an energy barrier, causing a charge spike. Internal circuits sense the charge spike and set the memory. If the electric field is removed from the crystal, the central atom stays in position, preserving the state of the memory. Therefore, the FRAM memory needs no periodic refresh and when power fails FRAM memory retains its data. It's fast, and doesn't wear out!
To increase the memory capacity, the cell size must always be reduced, and the design, process, and materials have been improved aggressively for this purpose. ferroelectric RAM products (FRAMs) are the most advanced of the flash challengers. The pioneer, Ramtron International Corp. (Colorado Springs, Colo.), has been selling FRAM chips since 1992. Their memory capacities are low, however, the largest being 256Kb—still a small fraction of the multimegabit chips offered by the major flash memory makers. In current commercial FRAMs, the interconnects that page link individual transistors into circuits are 0.5 µm wide and operate at 3 V. Narrower interconnects are desirable so that memory cells may be made smaller and be packed in greater numbers onto an IC. Ramtron's FRAMs are made by Fujitsu Ltd., Tokyo, which also sells its own FRAM products, mostly as embedded memory in microcontrollers and smart cards.
The biggest hurdle for FRAM developers is to advance the manufacturing technology to smaller geometries and lower voltages. R&D at Ramtron is aiming at 0.35-µm interconnect widths and 1.8-V operation. And last November, Texas Instruments Inc. (Dallas) announced that it had built a 64Mb FRAM in a standard 0.13-µm CMOS process, using technology licensed from Ramtron.
FRAM
INTRODUCTION:
A ferroelectric memory cell consists of a ferroelectric capacitor and a MOS transistor. Its construction is similar to the storage cell of a DRAM. The difference is in the dielectric properties of the material between the capacitor's electrodes. This material has a high dielectric constant and can be polarized by an electric field. The polarisation remains until it gets reversed by an opposite electrical field. This makes the memory non-volatile. Note that ferroelectric material, despite its name, does not necessarily contain iron. The most well-known ferroelectric substance is BaTiO3.
A Ferroelectric memory cell consists of a ferroelectric capacitor and a MOS transistor. Its construction is similar to the storage cell of a DRAM. The difference is in the dielectric properties of the material between the capacitor's electrodes. This material has a high dielectric constant and can be polarized by an electric field. The polarisation remains until it gets reversed by an opposite electrical field. This makes the memory non-volatile.Data is read by applying an electric field to the capacitor. If this switches the cell into the opposite state (flipping over the electrical dipoles in the ferroelectric material) then more charge is moved than if the cell was not flipped. This can be detected and amplified by sense amplifiers. Reading destroys the contents of a cell which must therefore be written back after a read. This is similar to the precharge operation in DRAM, though it only needs to be done after a read rather than periodically as with DRAM refresh.
FRAM is found mainly in consumer devices and because of its low power requirements, could also be used in devices that only need to activate for brief periods. FRAM allows systems to retain information even when power is lost, without resorting to batteries, EEPROM, or flash. Access times are the same as for standard SRAM, so there's no delay-at-write access as there is for EEPROM or flash. In addition, the number of write cycles supported by the FRAM components is nearly unlimited—up to 10 billion read/writes. FRAM combines the advantages of SRAM - writing is roughly as fast as reading, and EPROM - non-volatility and in-circuit programmability
FRAM Technology
When an electric field is applied to a ferroelectric crystal, the central atom moves in the direction of the field. As the atom moves within the crystal, it passes through an energy barrier, causing a charge spike. Internal circuits sense the charge spike and set the memory. If the electric field is removed from the crystal, the central atom stays in position, preserving the state of the memory. Therefore, the FRAM memory needs no periodic refresh and when power fails FRAM memory retains its data. It's fast, and doesn't wear out!
To increase the memory capacity, the cell size must always be reduced, and the design, process, and materials have been improved aggressively for this purpose. ferroelectric RAM products (FRAMs) are the most advanced of the flash challengers. The pioneer, Ramtron International Corp. (Colorado Springs, Colo.), has been selling FRAM chips since 1992. Their memory capacities are low, however, the largest being 256Kb—still a small fraction of the multimegabit chips offered by the major flash memory makers. In current commercial FRAMs, the interconnects that page link individual transistors into circuits are 0.5 µm wide and operate at 3 V. Narrower interconnects are desirable so that memory cells may be made smaller and be packed in greater numbers onto an IC. Ramtron's FRAMs are made by Fujitsu Ltd., Tokyo, which also sells its own FRAM products, mostly as embedded memory in microcontrollers and smart cards.
The biggest hurdle for FRAM developers is to advance the manufacturing technology to smaller geometries and lower voltages. R&D at Ramtron is aiming at 0.35-µm interconnect widths and 1.8-V operation. And last November, Texas Instruments Inc. (Dallas) announced that it had built a 64Mb FRAM in a standard 0.13-µm CMOS process, using technology licensed from Ramtron.
